New Horizons approaching Ultima Thule

The spacecraft has been given a “go” to take the optimal path past Ultima Thule, which is a Kuiper Belt object, a billion miles beyond Pluto. The encounter will be January 1, 2019.

Ultima Thule is the bright yellow spot in the middle. The 2 possible flyby distances for New Horizons are indicated by the 2 concentric circles. The mission has decided to fly along the closer path, toward the target point marked by an X. Read more about this image via Johns Hopkins.

NASA said last week it has given a “go” to its New Horizons spacecraft, the same craft that made humanity’s first-ever visit to Pluto in 2015. The craft is still speeding outward in our solar system. It’s now approaching its next target, a Kuiper Belt object a billion miles beyond Pluto, nicknamed Ultima Thule. NASA said that – after a three-week search – it has found no apparent obstacles in the spacecraft’s optimal path, no rings, no small moons, no potential hazards. Thus New Horizons is now aimed on that optimal path for its January 1, 2019, encounter with Ultima Thule.

It will be another first for New Horizons, the farthest planetary flyby in human history.

And now we know the spacecraft’s path will take it about 2,200 miles (3,500 km) from Ultima Thule, instead of a hazard-avoiding detour that would have pushed it three times farther out. The last opportunity to maneuver the spacecraft onto another trajectory was December 18.

New Horizons was launched in 2006 and had its historic encounter with Pluto in 2015. Now the craft is in the Kuiper Belt, preparing to encounter its 1st object in this region, Ultima Thule, on January 1, 2019. Image via NASA.

In the meantime, as New Horizons gets closer to Ultima Thule (which is officially designated 2014 MU69) the mission’s science team is puzzling over the light reflected from this object.

The spacecraft has been taking hundreds of images to measure Ultima’s brightness, but these recent measurements appear to be at odds with a 2017 observation, made when Ultima Thule covered (occulted) a star as seen from Earth. That 2017 observation suggested that Ultima Thule might be not one, but two bodies orbiting around each other. If there aren’t two objects there, the science team said in 2017, then this little Kuiper Belt object might have a pronounced elongated shape.

Now scientists aren’t sure what to think, since repeated observations over the past three months haven’t revealed the variations in brightness they’d expect from two bodies, or from a spinning elongated body. Those variations in brightness are what scientists call a light curve. New Horizons principal investigator Alan Stern of the Southwest Research Institute (SwRI) in Boulder, Colorado, commented in a statement:

I call this Ultima’s first puzzle – why does it have such a tiny light curve that we can’t even detect it? I expect the detailed flyby images coming soon to give us many more mysteries, but I did not expect this, and so soon.

There are various possible explanations, including that Ultima Thule’s rotation pole may be aimed right at or close to the incoming spacecraft. Or Ultima Thule may be surrounded by a cloud of dust that obscures its light and makes the observations difficult to read. University of Virginia’s Anne Verbiscer, a New Horizons assistant project scientist, suggested an even stranger possibility:

Ultima [may be] surrounded by many tiny tumbling moons. If each moon has its own light curve, then together they could create a jumbled superposition of light curves that make it look to New Horizons like Ultima has a small light curve.

While that explanation is also plausible, she added, it has no parallel in all the other bodies of our solar system.

New Horizons will make its historic close approach to Ultima Thule at 05:33 UTC (12:33 a.m. EST; translate UTC to your time) on January 1, 2019. Stern commented:

The spacecraft is now targeted for the optimal flyby, over three times closer than we flew to Pluto. Ultima, here we come!

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Ultima’s shape was measured in July 2017 as its silhouette passed in front of a star – what’s known as a stellar occultation. View an animated version of this occultation. Image via NASA/Johns Hopkins/SwRI.

For more information on the New Horizons mission, including fact sheets, schedules, video and images, visit: and

Follow the New Horizons mission on Twitter and use the hashtags #UltimaThule and #UltimaFlyby to join the conversation.

Live updates will be also available on the mission Facebook page.

Bottom line: The New Horizons spacecraft has been given a “go” to stay on the optimal path to Ultima Thule for its January 1, 2019, encounter.

Via Johns Hopkins here and here.



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